Axial and shear fracture strength evaluation of silicon microneedles

Abstract

This research aims to quantify fracture limits of hollow silicon microneedles due to axial and shear forces, and validate
their use on human skin. Needle failure due to axial and shear loads may be due to heterogeneous peak stresses within the
bulk material. Analytical determination of physical usage limits of microneedles can be misleading, as beam fracture models
do not translate well to the micro-scale. In this research, various sizes of hollow silicon microneedles were fabricated.
Their point of failure due to fracture was tested via application of axial and shear forces. Axial fracture of 36 gauge silicon
microneedles took place at about 740 gram-force. It was determined that the axial force experienced by microneedles during
insertion into human cadaver skin, is much lower than this critical limit, and thus needle failure due to breakage can thus
be generally attributed to shear forces. Fracture limits of microneedles due to shear forces were quantified; ranging from
about 36 gram-force for 36 gauge needles to about 275 gram-force for 33 gauge needles.

Full text

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TECHNICAL PAPER
Axial and shear fracture strength evaluation of silicon
microneedles
Puneet Khanna•Kevin Luongo•Joel A. Strom•
Shekhar Bhansali
Received: 17 July 2009/Accepted: 1 March 2010/Published online: 24 March 2010
? Springer-Verlag 2010
Abstract
hollowsiliconmicroneedlesduetoaxialandshearforces,and
validate their use on human skin. Needle failure due to axial
and shear loads may be due to heterogeneous peak stresses
withinthebulkmaterial.Analyticaldeterminationofphysical
usage limits of microneedles can be misleading, as beam
fracturemodelsdonottranslatewelltothemicro-scale.Inthis
research, various sizes of hollow silicon microneedles were
fabricated.Theirpointoffailureduetofracturewastestedvia
application of axial and shear forces. Axial fracture of 36
gauge silicon microneedles took place at about 740 gram-
force. It was determined that the axial force experienced by
microneedles during insertion into human cadaver skin, is
muchlowerthanthiscriticallimit,andthusneedlefailuredue
to breakage can thus be generally attributed to shear forces.
Fracture limits of microneedles due to shear forces were
quantified; ranging from about 36 gram-force for 36 gauge
needles to about 275 gram-force for 33 gauge needles.
This research aims to quantify fracture limits of
1 Introduction
Advanced bioengineered systems have the potential to
close the loop between diagnostic and therapeutic elements
employed inmoderntreatmenttechniques, thus
constituting a ‘‘smart’’ system. Many such systems will
involve the transfer of physical entities through the skin.
Traditional approaches used to collect biofluids or deliver
drugs through the skin include use of hypodermic needles.
Though utilitarian, this method causes undesirable pain and
excessive tissue trauma. This is particularly troublesome
when frequent administration of drugs is necessary (such as
administration of insulin). Furthermore, control of drug
delivery is highly approximate using macroneedles.
Microneedles are increasingly being sought out as an
alternative to the macroneedle. In recent times, numerous
types of microneedles have been fabricated and used for
transdermal drug delivery (Gardeniers et al. 2003;
McAllister et al. 2003; Martanto et al. 2004; Park et al.
2005; Gill and Prausnitz 2007), vaccine delivery (Griss and
Stemme 2003), fluid analysis and sampling (Brazzle et al.
1999), dialysis (Zahn et al. 2005) and cellular DNA
delivery (McAllister 2000) among other applications.
Silicon is a highly desirable material for microneedle
fabrication. Silicon allows for needles that are smaller than
metallic ones. However, silicon microneedles are relatively
fragile as compared to metallic microneedles, and their
physical robustness has not been completely verified. It is
imperative to know the fracture strength of microneedles in
order to ascertain their physical limits. Unfortunately, ana-
lytical analysis of beams does not translate to the micro-
scale,andthusstrengthassumptionscannotbemade.Needle
failure due to axial loads may be due to heterogeneous peak
stresseswithinthebulkmaterial.Usuallythesepeak stresses
are the cause of needle failure, and fracture or buckling
equationsbased onhomogeneityofthematerialconsistently
over-estimate the strength of needles (Davis et al. 2004).
Apart from axial fracture, shear fracture strength of
microneedles are also a vital parameter that needs to be
studied. Microneedle usage involves stresses related to the
Information contained in this article does not necessarily reflect the
position or the policy of the government, and no official endorsement
is inferred.
P. Khanna (&) ? K. Luongo ? J. A. Strom ? S. Bhansali (&)
University of South Florida, 4202 E. Fowler Ave., ENB 118,
Tampa, FL 33620, USA
e-mail: pkhanna@mail.usf.edu
S. Bhansali
e-mail: bhansali@eng.usf.edu
123
Microsyst Technol (2010) 16:973–978
DOI 10.1007/s00542-010-1070-4

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non-uniformity of skin contour, inadvertent slippage during
insertion or removal, and reasonable human movements
during periodofpenetration.These cause lateral forces tobe
applied on the needles. Breakage due to shear is a common
cause of needle failure. Though, there have been limited
studies on shear strength of silicon microstructures (Chen
etal.2000),shearstrengthofmicroneedleshavenotyetbeen
quantified. This research aims to validate the utility of sili-
con microneedles as a fluid delivery medium by performing
various fracture strength tests, and skin insertion tests.
2 Fabrication of silicon microneedles
In this research silicon microneedles were fabricated with
sizes ranging from 33 gauge to 36 gauge (respective sizes
are depicted in Table 1). The needles were fabricated using
a 4 in., 400 lm thick, h100i oriented, double side polished
silicon wafer. First, lithography was performed on the
backside of the wafer using photoresist AZ P4620 (spin at
500 rpm 10 s, 2,000 rpm 60 s; soft bake at 90?C for
20 min in oven; exposure for 55 s at 25 mW/cm2; develop
in 1:1 AZ400K:H2O for 4 min; hard bake at 90?C for
10 min). This step patterned the wafer with circular holes
outlining the through-wafer microfluidic channels for fluid
delivery. Next, DRIE (Alcatel AMS 100 SDE) was per-
formed on the backside of the wafer for 20 min, forming
approximately 200 lm deep holes (etch step, SF6 at
300 sccm with 3 s cycle; passivation step, C4F8 at
200 sccm and O2at 20 sccm with 1.4 s cycle; pressure,
5.25 9 10-1mTorr; source generator power, 2,400 W;
substrate holder power, used in pulsed mode, high cycle at
100 W for 25 ms, low cycle at 0 W for 75 ms; substrate
holder He pressure, 9.75 9 10-3mTorr; Bias voltage,
negative 185 V). Lithography was then performed on the
front side of the wafer, again using photoresist AZ P4620
(same recipe as before). This step outlined the microneedle
sidewalls. Next, DRIE of silicon was done for about
40 min to form the microneedles (same recipe as earlier
DRIE step). The etching ceases when through-holes are
formed due to helium leaking into the main chamber. A
2 min extended etch step was now performed with the
wafer kept over an aluminum chuck to ensure that
the needle bores open all over the wafer. Figure 1 depicts
the fabrication process of these microneedles. Figure 2
shows an SEM image of an array of these microneedles.
3 Experimental setup
In order to test the microneedle strength, 200 lm tall
needles were mounted onto a 0.5 in. 9 0.5 in. aluminum
block using double sided adhesive tape. The movement of
this aluminum block could be controlled via micromotor
actuators. A 0.5 mm wide stainless steel post was attached
to another aluminum block, which was in turn mounted
on a compression load cell (LCFA-500gF sensing capac-
ity, Omega Co.). The load cell measures force in ‘gram-
force’ (gf) units. The internal construction of the load cell
consists of a full four-arm Wheatstone bridge, capable of
producing repeatable measurements. The load cell was
interfaced to one channel load cell input, 16-Bit, RS-485
Table 1 Size of fabricated needles
Needle gauge (gauge)Inner diameter (lm)Outer diameter (lm)
33100 200
34 85 160
3565130
3630100
Fig. 1 Fabrication process of hollow silicon microneedles
Fig. 2 Silicon microneedle array
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Data Acquisition Module (Superlogics-8016) to obtain
real time results. The load cell was supported on a steel
block, mounted on a XYZ stage with two 1 lm resolution
manual micrometers screws (SM-13, Newport Co.).
The needle attachment block was supported on an XYZ
stage with motorized micrometers (Z600 series Thorlabs
Motorized Actuators) with 1 lm resolution. The motorized
actuators were controlled by a custom-built LabVIEW
program, which also displayed the real-time output from
the load cell. Real-time visual monitoring of the needle
insertion was done by a long-distance high magnification
microscope. Video capturing was done via a camera
mounted on the microscope. Figure 3 shows the micro-
needle testing setup.
Two types of strength testing was done—axial as well as
shear fracture. For axial fracture tests, needles were aligned
in line with the axis of the steel post, while for shear
fracture tests, the needles were aligned perpendicular to the
post. In both tests, a single microneedle was made to travel
towards the rigid steel post. As the needle presses against
the post, the incident force on it increases, eventually
resulting in its fracture. The micrometer actuators allow for
precise control of the needles’ and the post’s position. In
order to obtain reliable force data, the microneedles were
moved at a slow rate of 50 lm/min. The force output from
the load cell and the distance travelled by the micrometer
actuator were recorded via the data acquisition system. By
visual monitoring, the steel post was aligned along indi-
vidual microneedles, one at a time. For shear fracture tests
care was taken to align the post as high as possible for
successful needle breakage. This is because the measured
shear force increases if the point of lateral force is lowered.
The experiments were recorded on video and the needle
collapse was observed via a live feed. Force data was
captured simultaneously. Figure 4 shows a schematic of
the fracture test setup.
In order to verify the feasibility of using silicon mi-
croneedles for insertion into skin, insertion force tests into
human cadaver skin was performed. The setup was similar
to that used for fracture testing, except that the rigid post
was replaced with an aluminum block on which skin was
mounted using double sided adhesive tape. A 4 9 4 array
of the 36 gauge microneedles was used. The skin was
previously preserved at -80?C. The skin was loaded on the
load cell immediately after bringing it to room temperature.
At this point, and the reading from the load cell was
allowed to settle down for a few minutes. For these
experiments too, to ensure reliable data, the microneedles
were inserted at a slow rate off 50 lm/min into the skin.
The force output from the load cell and the distance trav-
elled by the micrometer actuator were recorded via the data
acquisition system. The test was repeated multiple times to
observe for any breakage of needles. To validate successful
penetration into the test was repeated with a control sample
containing no microneedles.
4 Results
Axial fracture testing was first done on 36 gauge hollow
silicon microneedles. The microneedle is seen to collapse
completely at its critical load. There was no bending of
the needle observed. The needles collapsed completely
from the base, with no stub remaining. Figure 5 shows a
microneedle aligned axially with the steel post. The inset
shows the outcome after the needle fracture. Figure 6
shows the plot of force measurement versus distance
travelled by the microneedles. The peak incident force
corresponds to the instant of needle collapse. The graph has
slight non-linearity seen during rise in the incident force.
This is probably due to slight inadvertent slippage of the
needle over the steel post, or due to minimal buckling of
the needle itself. It should be noted that the X-axis of the
graph denotes distance traveled by the microneedles from
an arbitrary starting position near to the post.
The axial fracture tests were repeated for multiple
needles, and the peak forces resulting in fracture were
Fig. 3 Microneedle testing setup using a force measurement load cell
and motorized actuators
Fig. 4 Schematic of fracture test setup, a axial fracture test, b shear
fracture test
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tabulated. Table 2 shows the results of these axial fracture
tests. The axial strength of the smallest fabricated needles
was at the upper range of the load cell failure rating. The 35
gauge needles did not fracture within the safe usage limit of
the load cell. Strength testing on the 33 and 34 gauge
needles was not attempted since their strength rating was
expected to be much higher.
For the shear fracture tests, viable results were obtained
for all sizes of fabricated microneedles (33 gauge to 36
gauge). Graphs were plotted for each of the tests, and the
peak force noted. Figure 7 shows a microneedle aligned
perpendicular to the steel post for these tests. Figure 8
shows a typical plot obtained from the shear fracture tests.
The particular graph shown is of a test performed on 33
gauge needle. Here too, the point of force drop represents
the instant when the needle breaks. The results of shear
testing are shown in Table 3. A graph for average shear
force was plotted for various needle dimensions such as
inner diameter, outer diameter, wall thickness and radius.
This is shown in Fig. 9.
The skin insertion tests resulted in successful penetra-
tion of microneedles into skin. It was observed that even
after multiple repetitions of the insertion test into different
Fig. 5 Microneedle aligned with steel post for axial testing. Inset
microneedle completely collapses with no stub remaining
Fig. 6 Typical result of axial fracture test on 36 gauge microneedle
Table 2 Axial fracture strength of silicon needles
Test no. Axial fracture strength (gf)
36 Gauge needle 35 Gauge needle33 and 34 Gauge
1 661.85 Out of rangeTest not performed
2 874.47 Out of range
3682.49Out of range
Mean 739.6 N/AN/A
Fig. 7 Steel post aligned perpendicular to silicon microneedle for
shear strength testing
Fig. 8 Typical result of shear fracture test
Table 3 Shear fracture strength of silicon needles
Test
no.
Shear fracture strength (gf)
33 Gauge
needle
34 Gauge
needle
35 Gauge
needle
36 Gauge
needle
1262.18 186.984.98 36.77
2 255.44199.6884.59 38.29
3 308.05170.42 70.1431.77
Mean 275.22185.67 79.935.6
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skin samples, all microneedles remained intact. Figure 10
shows intact needles after repeated insertions into skin.
Figure 11 depicts a typical graph of monitored force versus
distance travelled by the microneedles during these tests.
During microneedle insertion, the skin initially resists the
penetration of the needles. This is observed as a rise in the
force output from the load cell. At the point of microneedle
insertion, the stratum corneum is ruptured and the needles
undergo a sudden movement into the skin. This movement
is translated by the load cell as a corresponding drop in the
measured force. The discontinuity in the insertion force
graph thus corresponds to the instance of insertion into
skin, and provides an analytical proof of successful pene-
tration. Repeated insertion tests resulted in similar plots, all
containing a single discontinuity.
In order to confirm that the force drop observed was
indeed due to presence of needles, a control experiment
was carried out. In this case, a square piece of silicon wafer
of the same size as the microneedle array was used instead,
and pressed against the skin. Figure 12 shows a typical
result obtained from the control experiment. The graphs
obtained from the control experiments overlap with high
degree of agreement with the microneedle insertion tests.
This shows that the skin deformation dynamics remain
similar in both cases.
These results confirm that the axial force experienced by
the 36 gauge microneedles during insertion into skin is low
enough to enable reliable and repeated usage without any
concern for axial fracture. Since the axial strength of mi-
croneedles larger than 36 gauge (35 gauge to 33 gauge) can
be expected to be even higher, it is safe to assume that axial
fracture limits of larger needles are not a critical factor for
their usage on human skin.
5 Conclusion
In this research, various sizes of hollow silicon micronee-
dles were fabricated. Their point of failure due to fracture
was tested via application of axial and shear forces. Suc-
cessful penetration into skin was verified by insertion tests
into human cadaver skin. The fracture limits of silicon
microneedles due to axial forces was quantified. Axial
fracture of 36 gauge silicon microneedles took place at
about 740 gram-force. It was shown that the axial force
experienced by microneedles during insertion into skin, is
much lower than this critical limit. Since the axial strength
of microneedles larger than 36 gauge can be expected to be
even higher, fracture due to purely axial compression
Fig. 9 Shear force variation for various needle dimensions
Fig. 10 Visual monitoring of microneedles after insertion into skin
Fig. 11 Insertion force of silicon microneedles into cadaver skin
Fig. 12 Skin insertion control test without microneedles
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during insertion into skin can be ruled out. Needle failure
due to breakage can thus be generally attributed to shear
forces. Fracture limits of microneedles due to shear forces
were quantified. The shear strength of needles ranged from
about 36 gram-force for 36 gauge needles to about 275
gram-force for 33 gauge needles.
Acknowledgments
of the Army under Award Number W81XWH-05-1-0585. The U.S.
Army Medical Research Acquisition Activity (820 Chandler Street,
Fort Detrick, MD 21702-5014) is the awarding and administering
acquisition office.
This work was funded by the U.S. Department
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